The extraction process.
"Different names for olive oil indicate the degree of processing the oil has undergone as well as the quality of the oil. Extra virgin olive oil is the highest grade available, followed by virgin olive oil. The word "virgin" indicates that the olives have been pressed to extract the oil; no heat or chemicals have been used during the extraction process, and the oil is pure and unrefined. Virgin olive oils contain the highest levels of polyphenols, antioxidants that have been linked with better health."
Note that Southern European countries, Italy, Greece, Spain and Portugal produce the highest grade of oil.
There are 2 main processes to cold extract the oil, centrifuge (lower grade)and pressed (the highest grade).